This invention relates to a sample hold apparatus for a key signal in an electronic musical instrument.
An apparatus of this kind as already known in the art is shown in FIG. 1. A keyboard circuit 1, which generates a voltage corresponding to a depressed key, is connected at its output terminal 1a to an input terminal of a comparator 2. An output terminal of the comparator 2 is connected to a memory condenser 4 and a buffer circuit 5 through a gate 3. An output terminal of the buffer circuit 5 is connected to another input terminal of the comparator 2. The keyboard circuit 1 is so constructed that a plurality of series resistances 8, 8 . . . are connected in series to an electric power source terminal 6 through a constant-current circuit 7, and a plurality of keyswitches 10,10 . . . which are closed by depression of respective keys and are connected to respective connecting points of the resistances 8,8 . . . These key-switches 10,10 . . . are connected together at their movable contacts and are connected in common to the output terminal 1a. Key-switches 11,11 . . . for generating respective keying signals are located on one side of the key-switches 10,10 . . . . These key-switches 11, 11 . . . are connected together at their stationary contacts and connected in common to an electric power source terminal 12. They are also connected together at their movable contacts and connected in common to a control electrode 3a of the gate 3. The output terminal of the buffer circuit 5 is also connected to a voltage-controlled oscillator 13 (hereinafter called "VCO 13"). An output terminal thereof is connected to a speaker 17 through a voltage-controlled filter 14 (hereinafter called "VCF 14"), a voltage-controlled amplifier 15 (hereinafter called "VCA 15") and an amplifier 16.
Additionally, a single common output terminal 11a of the foregoing key-switches 11, 11 . . . is connected to control electrodes of the VCF 14 and the VCA 15 through an envelope signal generating circuit 18 (hereinafter called "ADSR 18").
Thus, if a key is depressed, a voltage corresponding to the depressed key is generated at the output terminal 1a of the keyboard circuit 1 and a keying signal is obtained at the common output terminal 11a of the key-switches 11, 11 . . . . The individual key switches 10 and 11 are ganged. As a result, the gate 3 is opened and the memory condenser 4 is charged so that the two input terminals of the comparator 2 may become equal in potential, and the VCO 13 oscillates with a frequency corresponding to an output voltage of the buffer circuit 5. Meanwhile, the foregoing keying signal drives the ADSR 18 so that an output signal thereof may control the VCF 14 and the VCA 15, and as a result a musical tone signal having an envelope is obtained from the speaker 17.
The ADSR 18, as is well known, generates a voltage waveform A (an envelope signal) as shown in FIG. 2. As will be clear from this waveform A, it has a release time beginning at the moment when the key is released (key off), and thus the musical tone becomes a natural attentuated one. In view of this fact, it is thought necessary that the memory condenser 4 is kept at a properly or charged potential even after the key is released. Accordingly, to achieve this, the key-switches 10, 10 . . . and 11,11 . . . must be set so that when the key depression is released, the key-switches 11,11 . . . are opened earlier than the key-switches 10,10 . . . . Additionally, in this case, the time difference between the key-switches 10,10 . . . and the key-switches 11,11 . . . must be as small as possible, for instance, less than the ratio in which the numerator is 1 and the denominator is several tenths of a sec. Such a setting, however, is extremely difficult, and it often happens that the time difference becomes too large or becomes zero or the key-switches 10,10 . . . are opened earlier. As a result, therefore, the musical tone is deformed.
It is therefore an object of the present invention to provide an arrangement which is free of the disadvantages described above.
Another object of the present invention is to provide an arrangement of the foregoing character, which is simple in construction and may be economically maintained in service.
A further object of the present invention is to provide an arrangement, as described, which has a substantially long operating life.